The present invention relates to a device for detecting zero crossings of an electrical signal.
Zero crossing detectors are well known in the art and are used, for example, in synchronizing digital circuits to a sinusoidal reference signal. In a typical application, a sinusoidal signal produced by the power network or by an oscillator is converted into a square wave. It is important, in many cases, that the square wave shift from one logic level to the other precisely in time with the zero crossings of the sinusoidal signal.
In one particular application used in telephone networks, a sinusoidal signal is generated at an exchange to ring a subscriber telephone. It is desirable to connect this ringing signal to the subscriber telephone line when the ringing voltage is zero, so as to avoid an audible "click" at the telephone station, and to switch off the ringing signal at zero current so as to avoid a current surge through the switch. Thus, the ringing signal switch for each subscriber line must be synchronized with the positive-going and negative-going zero crossings of the ringing signal.
Conventionally, a zero crossing detector comprises essentially a threshold detector or "Schmitt trigger" which continuously compares the electrical signal with a reference voltage. This reference voltage is set at the DC level (for instance, zero volts) of the incremental electrical signal. The threshold detector produces a logical "one" when the electrical signal is above the threshold and a logical "zero" when the electrical signal is below the threshold. This type of threshold detector is adequate for most situations; however, inaccuracies in the voltage comparison and variations in the bias level of the signal result in inaccuracies in the zero crossing detection. Furthermore, since the threshold detector responds to change the logic level only after the voltage level of the zero cross (reference voltage level) has been crossed with a certain degree of tolerance, there is an inherent delay in the output of the zero crossing detector.